In a motor vehicle with a front mounted engine, it is known to secure the engine to the vehicle body in the engine compartment using mounts that allow some relative movement between the engine and the body. Though the term “engine mount” is used herein, in some vehicle configurations, the mount supports the entire drive train, as the engine, transmission and differential may be formed as one unit. Each mount has a rigid main body that is secured to the body of the vehicle and a fixing point from which the engine is suspended that is connected to the main body by bushings, and sometimes fluid filled shock absorbers, that serve to isolate the body from engine noise and vibrations.
The engine mounts generally are secured to two frame rails (usually extending in the longitudinal direction), or to a sub-frame of the vehicle, and are positioned one on each side of the engine compartment. Apart from supporting the power train, the frame rails form part of the monocoque frame that ensures the structural rigidity of the vehicle body and longitudinal frame rails usually are intentionally designed to crumple in the event of a front end collision, in order to absorb the energy of the impact.
The front bumper of the vehicle is mounted on the front ends of the rails and is itself intended to absorb the energy of a minor impact (below, say, 10 kph) without the frame rails incurring any damage. In the event of a more serious impact the front ends of the frame rails crumple, but only as far as the point to which the engine mount is secured. This is because the rigid main body of the engine mount strengthens the frame rail and prevents it from being crushed. In this way, even for moderate impacts (below, say, 30 kph), the engine is not pushed towards the passenger compartment. At still higher impact speeds, the section of the frame rail behind the engine mounts deforms to absorb still more energy.
The main body of the engine mount is secured to the frame rail by fixing bolts at the front and rear of the engine mount. The section of the frame rails between the fixing bolts of the engine mount cannot be used to the same extent in absorbing the energy of a collision. It would therefore be desirable to minimize the distance between the fixing bolts of the mount but this distance is dictated by other factors, amongst them the size of the bushings needed to provide the desired absorption and the size of the fluid filled hydraulic damper.